The present invention relates to an optical scanner for an image forming apparatus.
An optical scanner having a plurality of mirrors for scanning a document is extensively used with image forming apparatuses. The scanner has focusing means therein and is combined with an image forming apparatus having a photoconductive element. The scanner focuses a document image onto the photoconductive element via the focusing means and thereby forms a latent image. The latent image is developed to turn out a corresponding toner image. The toner image is transferred to a paper and then fixed thereon. The scanner forms a part of an analog or digital copier or similar image forming apparatus and includes mirrors which are movable along a document during the course of scanning. A drive source for driving the movable mirrors is implemented by a DC servo motor in a high-speed machine or by a stepping motor in a medium- or low-speed machine. A stepping motor is commonly used because it is advantageous in the cost aspect.
However, the problem with a stepping motor is that it noticeably vibrates and reaches the maximum vibration level at a particular period (frequency). In a copier having a broad range of magnification change ratios, the vibration of the motor causes the mirrors to resonate or even to deform themselves. Various anti-vibration approaches have heretofore been proposed, as follows:
(1) a vibration absorber mounted on a scanning unit and having a variable spring constant (Japanese Patent Laid-Open Publication No. 3-144626);
(2) an anti-vibration member intervening between a mirror and a support frame in order to preserve the planeness of the mirror while protecting it from resonance (Japanese Patent Laid-Open Publication No. 3-180867);
(3) a support member supporting the opposite ends and intermediate portion of a mirror in order to prevent the mirror from vibration (Japanese Patent Laid-Open Publication No. 4-149540);
(4) a lug and a screw respectively supporting the front and the rear of a mirror in order to control vibration (Japanese Patent Laid-Open Publication No. 4-282626);
(5) a rigid member, aluminum material, or zinc-processed steel plate adhered to the rear of a mirror in order to control vibration; and
(6) a motor having a heavy output shaft.
However, the above scheme (1) results in a complicated construction. The schemes (2)-(4) each supports only a part of a mirror and, therefore, deteriorates the planeness of the mirror. The scheme (5) adheres a member whose planeness is uncertain to a mirror which needs high planeness. Hence, even the scheme (5) is apt to impair the planeness of the mirror although it may control vibration. Further, the scheme (6), relying on a heavy motor output shaft, increases the torque and, therefore, the size of the machine. This obstructs the miniaturization of the apparatus and cost reduction.
Japanese Patent Laid-Open Publication No. 2-253274 and Japanese Utility Model Laid-Open Publication No. 62-42389 also disclose implementations relating to an optical scanner.